1. Field of the Invention
This invention relates to a battery pack including a protection circuit which detects an overcharge, an overdischarge or an overcurrent of a secondary battery and turns off a switching element arranged in wiring between a load and the secondary battery or between a charging device and the secondary battery.
2. Description of the Related Art
In recent years, a portable device, such as a digital camera, carries lithium ion batteries as secondary batteries. Because lithium ion batteries are prone to overcharging or overdischarging, the lithium ion batteries are arranged with a battery pack containing a protection circuit for protecting the batteries from overcharging or overdischarging.
FIG. 10 shows the composition of a battery pack 1 according to the related art. In the battery pack 1 of FIG. 10, a lithium ion battery 2 is connected in parallel to a series circuit in which a capacitor C1 and a resistor R1 are connected in series. A positive electrode of the lithium ion battery 2 is connected to an external terminal 3 of the battery pack 1, and a negative electrode of the lithium ion battery 2 is connected to an external terminal 4 of the battery pack 1 via a pair of n channel MOS (metal oxide semiconductor) transistors M1 and M2 for current interception.
The drains of the MOS transistors M1 and M2 are connected to each other, the source of the MOS transistor M1 is connected to the negative electrode of the lithium ion battery 2, and the source of the MOS transistor M2 is connected to the external terminal 4. Body diodes D1 and D2 are connected between the drain and the source of each of the MOS transistors M1 and M2 in an equivalent manner.
A protection IC (integrated circuit) 5 is arranged in the battery pack 1 so that the protection IC 5 includes an overcharge detecting circuit, an overdischarge detecting circuit, and an overcurrent detecting circuit. A power-source voltage Vdd is supplied to the protection IC 5 through the resistor R1 from the positive electrode of the lithium ion battery 2, and a power-source voltage Vss is supplied to the protection IC 5 from the negative electrode of the lithium ion battery 2, so that the protection IC 5 operates.
When an overdischarge or overcurrent is detected by the overdischarge detecting circuit or the overcurrent detecting circuit, the protection IC 5 sets the output signal DOUT to a low level to turn the MOS transistor M1 to an OFF state. When an overcharge is detected by the overcharge detecting circuit, the protection IC 5 sets the output signal COUT to the low level to turn the MOS transistor M2 to the OFF state.
FIG. 11 shows the composition of a battery pack 1 according to the related art. In the battery pack 1 of FIG. 11, a thermistor R3 is further arranged in the battery pack 1 in addition to the elements shown in FIG. 10. One end of the thermistor R3 is connected to a terminal 6 of the battery pack 1 and the other end of the thermistor R3 is connected to the external terminal 4.
A predetermined voltage from a charging device is applied to the terminal 6 of the battery pack 1 through a voltage divider resistor in order to charge the battery pack 1. The resistance of the thermistor R3 changes in accordance with a temperature of the battery pack 1, and the voltage of the terminal 6 changes accordingly. The voltage of the terminal 6 is detected and if a temperature of the battery pack 1 derived from the detected voltage exceeds a predetermined value, the charging device is controlled to stop the charging of the battery pack 1.
Japanese Laid-Open Patent Application No. 2004-152580 discloses a battery pack in which a first diode arranged in a first direction (charging direction) and connected in series to a temperature protection element (PTC element) and a second diode arranged in a second direction (discharging direction) opposite to the first direction and connected in parallel with the first diode and the temperature protection element (PTC element) are connected to a secondary battery. This battery pack is arranged so that, when normal discharging of the secondary battery is performed, a current flows only through a channel including the second diode in the second direction and does not flow through a channel including the first diode in the first direction. Thus, even when a temperature of the battery pack becomes high at the time of normal discharging, no current flows through the channel including the first diode and it is possible to inhibit operation of the temperature protection element (PTC element).
The battery pack according to the related art, shown in FIG. 10, does not have a temperature protection function to protect the battery pack from high temperature. The battery pack according to the related art, shown in FIG. 11, has a temperature protection function to protect the battery pack from high temperature. However, in this battery pack, the predetermined voltage from the charging device is supplied through the voltage divider resistor. In a case where the predetermined voltage of the charging device changes remarkably, or in a case where the resistance of the voltage divider resistor has a significant error, it is difficult to detect a temperature of the battery pack accurately, which hinders the battery pack from performing appropriate charging control.